The present invention relates to an inkjet recording apparatus, in particular, to an inkjet recording apparatus characterized by enhanced feed precision in moving a recording medium in the direction of feed.
In an inkjet recording apparatus for recording a desired image by discharging ink particles on a recording medium, the recording head for discharging ink particles is moved on the recording medium in the main scanning direction, and the recording medium is moved in the sub-scanning direction orthogonal to the main scanning direction every time one line has been recorded. This procedure is repeated.
In the prior art, movement of a recording medium in the sub-scanning direction is carried out in an intermittent feed by a stepping motor, or by a DC motor equipped with a rotary encoder. In the former case, a predetermined number of pulses is added to the stepping motor, every time the recording head has recorded one line in the main scanning direction, and the recording medium is moved a predetermined distance in the sub-scanning direction, using the number of the steps at that time as a feed distance (Patent Document 1). In the latter case, the amount of movement of the recording medium fed in the sub-scanning direction is scanned as the pulses of the rotary encoder, and the pulses capable of obtaining a predetermined amount of feed is counted, whereby control is performed (Patent Document 2).
According to a further prior art, a rotary encoder is mounted on the rotary shaft of the feed roller for moving the recording medium in the sub-scanning direction, and the pulses are counted, whereby the amount of feed is controlled (Patent Document 3).
In recent years, there has been an increase in the number of recording head nozzles due to high speed recording of a high quality image. With the growing length along the nozzle train of the recording head, the amount of feed of the recording medium in the sub-scanning direction is increased, and this requires improvement of feed precision. For example, in the prior art method of filling the image by n-passes of the nozzle interval, efforts have been made to find out a method for getting a high-quality image free of banding by so-called block printing, where movement of a very short distance is performed in n−1 passes and movement of a long distance is carried out in the remaining one pass. Such a recording method requires the recording medium to be moved a long distance at a time in the sub-scanning direction. This, in turn, requires a drastic improvement of feed precision. Accordingly, such a printing method is not used in practice.
This is because of the following reasons: As described above, in the inkjet recording apparatus, the amount of recording medium feed is detected only indirectly by counting the pulses of the stepping motor for driving the recording medium feed roller, and the number of pulses of the rotary encoder. This method produces an error with reference to the actual amount of the recording medium feed. This error causes a white streak to be produced especially between the blocks, whereby image quality is deteriorated. In other words, the amount of recording medium feed detected by counting the pulses fails to represent the true amount of the recording medium feed, due to many factors such as a feed roller diameter, feed roller shaft center position, the difference in the thickness of the recording medium and slip between the feed roller and recording medium, regardless of whether a stepping motor or a DC servo motor using a rotary encoder is used. This produces an error with reference to actual amount of the recording medium, thereby causing a white streak to be produced. Such a trouble tends to be improved when a rotary encoder is mounted on the rotary shaft of the feed roller, but no sufficient improvement effect has been obtained so far in the face of the increasing feed amount.
To solve this problem, a proposal has been made of a technique wherein a predetermined mark is recorded on the recording medium and the amount of recording head movement is determined with reference to the position where this mark is detected, thereby improving the recording medium feed precision. (Patent Documents 4 and 5)
[Patent Document 1] Official Gazette of Japanese Patent Tokkaihei 11-334160
[Patent Document 2] Official Gazette of Japanese Patent Tokkaisho 59-171664
[Patent Document 3] Official Gazette of Japanese Patent Tokkaihei 4-19149
[Patent Document 4] Official Gazette of Japanese Patent Tokkaihei 3-42264
[Patent Document 5] Official Gazette of Japanese Patent Tokkai 2000-218891
The present inventors have proposed an inkjet recording apparatus wherein ink particles are discharged from at least one nozzle in the process of moving the recording head in the main scanning direction, and a predetermined mark is recorded on the recording medium and is detected by a mark detecting means that moves the mark together with the recording head, in such a manner that the amount of recording head movement is determined with reference to the position where the mark detecting means has detected a detection signal, with the result that high precision movement is ensured even when the recording medium is moved a long distance (Tokugan 2002-304279).
In such an inkjet recording apparatus, a mark detecting means for detecting a mark recorded on the recording medium is arranged in the vicinity of the recording head, and is fed in the main scanning direction together with the recording head. It performs reciprocating motion by reversing its direction on the side of the recording medium along the width. Reversing its direction on the side of the recording medium along the width is essential to move the recording head at a constant speed on the printing area of the recording medium. This requires a certain accelerated feed distance until the constant speed is reached from the rest position at 0 speed.
In the meantime, the mark recorded by the recording head for recording an image is basically recorded in the vicinity of the lateral end of the recording medium deviated from the printing area of the recording medium. Thus, to detect the mark, the movement of the recording head must be stopped temporarily at the position of the mark recorded on the recording medium.
However, the mark detecting means is located above the recording medium, and the recording head rest position for detecting the mark by means of the mark detecting means is not located at the position where the cartridge with the recording head mounted thereon is reversed to perform reciprocating motion. Therefore, the cartridge restarts movement from the position where the mark detecting means has detected the mark, until it reaches the reversing position. Then the reversing operation must be started at that position. Thus, the cartridge is required to repeat start/stop operations for mark detection, in addition to reversing operations for reciprocating motion. This is a waste of movement, and hence reduction in printing productivity.